Add ifdef around variable in tracking.F90
Add ifdef around variable in tracking.F90

johnnyliu27/openmc,mit-crpg/openmc,johnnyliu27/openmc,smharper/openmc,liangjg/openmc,johnnyliu27/openmc,mit-crpg/openmc,smharper/openmc,walshjon/openmc,liangjg/openmc,amandalund/openmc,shikhar413/openmc,smharper/openmc,paulromano/openmc,amandalund/openmc,wbinventor/openmc,paulromano/openmc,liangjg/openmc,amandalund/openmc,walshjon/openmc,johnnyliu27/openmc,shikhar413/openmc,wbinventor/openmc,smharper/openmc,amandalund/openmc,mit-crpg/openmc,walshjon/openmc,liangjg/openmc,paulromano/openmc,mit-crpg/openmc,walshjon/openmc,wbinventor/openmc,shikhar413/openmc,shikhar413/openmc,paulromano/openmc,wbinventor/openmc